1. Field of Invention
The invention relates to seed crystals and the seed chucks used to hold the seed crystals during growth. More particularly, the invention relates growing dislocation-free (DF) crystals.
2. Description of Related Art
The monocrystalline silicon that is the starting material for many semiconductor electronic components is commonly prepared by a Czochralski (CZ) process. In this process, pieces of polycrystalline silicon are placed in a crucible and melted to a liquidous state, thereby creating a melt. A seed crystal having the desired monocrystalline atomic structure is then lowered into contact with the molten silicon. As the seed crystal is slowly extracted from the melt, a monocrystalline crystal is drawn from the melt having the same atomic structure as the seed crystal. One such type of crystal pulling apparatus is disclosed in EP 783 047, which is incorporated by reference herein.
Unfortunately, dislocation defects are generated in the seed crystal due to thermal shock as the seed crystal contacts the relatively hot melt. If corrective actions are not taken, the dislocation defects propagate through and multiply in the growing crystal. As known to those skilled in the art, dislocations generally propagate along crystallographic planes. For a silicon seed crystal having a &lt;100&gt; orientation, the dislocations typically propagate along a plane that extends at an angle of 55.degree. from the longitudinal axis of the crystal.
In order to terminate the dislocations prior to propagation through the main body of the crystal, crystals typically have a neck section extending between the seed crystal and the main body of the crystal. The most common method of eliminating dislocations is known as the Dash method and involves growing a neck having a relatively small diameter and a relatively long length. For example, a neck grown according to the Dash method may have a diameter of between 2 mm and 4 mm and a length between 30 mm and 200 mm. As the neck is grown, the dislocations propagate through the neck toward the interface of the seed crystal and the melt. As a result of the extended length and small diameter of the neck, however, the dislocations terminate at the exterior surface of the neck such that the main body of the crystal is dislocation-free (DF). The crystal is then expanded in diameter through the shoulder portion to the DF main body. Since there is no easy and reliable method to determine if the dislocations have been terminated, the Dash method generally requires the neck to have a relatively small diameter and an extended length in order to effectively terminate most, if not all, dislocations.
Although the Dash method is widely utilized, the Dash method has several significant disadvantages. For example, the time and expense associated with growing the neck section are non-recoverable since the neck is ultimately discarded as waste. Also, since the entire crystal is supported during growth by the relatively thin neck section, the maximum mass of a crystal is limited, typically to approximately 140 kg. Although this weight limit poses productivity and economic problems for crystals having conventional diameters of 150 mm or 200 mm, even more problems are created by this weight limit as the silicon industry begins to investigate and grow crystals having diameters of 300 mm or more.
To reduce the thermal shock, dipping speed has also been carefully controlled. Often the seed crystal will be lowered toward the melt and held slightly above the melt until the temperature of the seed crystal stabilizes. Such delay reduces efficiency and adds significant time to the process. Further external gases flowing toward the seed crystal cool the seed crystal, thus making it difficult to stabilize the temperature of the seed crystal.
Therefore, notwithstanding prior technique to grow DF crystals, a need still exists for an improved technique for growing DF crystals. In particular, a need exists for improved techniques for growing relatively large and heavy DF crystals without subjecting the neck of the crystal to excessive force and without repeatedly adjusting the pulling speed or requiring additional equipment for lifting or otherwise supporting the crystal during growth.